The present invention relates generally to an improved power amplifier and, more particularly, to a multi-output power amplifier and to a multi-way speaker system including such amplifier.
A typical power amplifier is shown in FIG. 1. This amplifier consists of two amplifier stages: a voltage amplifier 50 followed by a current: amplifier 53, the input of which is connected to the output of the voltage amplifier. A feedback resistor 52 is typically connected between the input of the voltage amplifier 50 and the output of the current amplifier 53 to reduce distortions caused by nonlinearity and/or variations in frequency response in the amplifier.
Power amplifiers of this type are often used to drive multi-way audio speaker systems. Two typical two-way speaker configurations using power amplifiers of the type described hereinabove are shown in FIGS. 2 and 3. The two-way speakers of FIGS. 2 and 3 are comprised of an amplification section (including either one or two amplifiers), a woofer 25 for filtering out high frequencies and emitting the low frequency components of the amplified signal, and a tweeter 30 for filtering out low frequencies and emitting the high frequency components of the amplified signal. The "high" and "low" frequencies can be selected by choosing appropriate values for the components of the woofer and tweeter filters.
FIG. 2 illustrates a "bi-cable" amplification arrangement in which a signal is applied to the input of a single power amplifier 15 and the amplified signal is split between two cables 35, 36 at the amplifier output 28. The signals are then delivered to the woofer 25 and tweeter 30. FIG. 3 illustrates a "bi-amp" amplification arrangement in which a signal is applied at the inputs of two amplifiers 17, 20. The output of one amplifier 17 is connected to the woofer 25 via a cable 35 and the output of the other amplifier 20 is connected to the tweeter 30 via a cable 36. Each of the two prior art arrangements depicted in FIGS. 2 and 3 has serious drawbacks.
In FIG. 2, the connection of more than one speaker component (i.e., a woofer 25 and a tweeter 30) to a single amplifier 15 will often result in undesirable and unpredictable distortions caused by the interaction of the different reactances of the output: load, in this case the speaker components. This problem may be avoided by the configuration shown in FIG. 3, which employs multiple power amplifiers to ensure isolation among the speaker components. However, while using a separate power amplifier for each speaker component provides a high degree of isolation, disparate signals will often be delivered to each component due to the inherent differences in "like" electrical components. This is a particularly undesirable feature for high fidelity audio systems. Having a separate amplifier for each speaker component also adds considerable expense to a speaker system.
Thus, there is a need for a multi-output amplifier which sufficiently isolates one output from the other while maintaining relative uniformity among the output signals.